The formation of propylene oxide-carbon dioxide (PO—CO2) copolymers has been widely investigated with a number of catalysts being evaluated for the production of these materials. The major thrust of the work to date has been to provide a method to convert a greenhouse gas into a useful product. The success of these studies has been limited because a majority of the catalysts require relatively long reaction times and high catalyst loadings. Double metal cyanide catalysts show the greatest potential because of the high yields and relatively fast rates of reaction that are characteristic of such catalysts. A disadvantage of using the DMC catalysts is the production of mixed polyether polycarbonates instead of alternating monomers of pure polycarbonates. Another disadvantage is that DMC catalysts also produce significant amounts of by-product cyclic alkylene carbonates (according to the following equation).

This formation of smaller amounts of by-product alkylene carbonate has been shown in several patents and publications. The amount of cyclic carbonate is not given in some of those patents, so it is necessary to make some assumptions based on the similarity of catalysts for which data is reported.
For example, Kuyper et al., in U.S. Pat. No. 4,826,953, disclose the use of DMC catalysts for the production of polyether polycarbonates using complexes based on zinc hexacyanocobaltates with glyme as the ligand. In the '953 patent, Kuyper et al. did not list the amount of cyclic carbonate produced, but based on the data from Table 1 of the '953 patent, the calculated amounts of cyclic carbonate appear to range from about 13% to about 31% (see table below).
Data Based on Table 1 of U.S. Pat. No. 4,826,953*CyclicCyclicRunProductPolyolcarbonatecarbonate (%)11627.81419.4208.41321694.21352342.22031781.51234.6546.93141732.81297.9434.92551782.91260.5522.42961772.31345.2427.12471777.31317.1460.226*Assumes that propylene carbonate formed is the difference between product yield and polyol yield as discussed therein at col. 6, line 62.
The formation of the cyclic carbonates reduces yields and may increase process costs due to the loss of raw material and the increased processing efforts necessary to remove the cyclic alkyl carbonates. As those skilled in the art are aware, if the cyclic carbonates are allowed to remain in the product and the linear carbonate is converted to polyurethane, the cyclic carbonates act as plasticizers and modify product properties. The catalysts used in the '953 patent were based on zinc hexacyanocobaltate complexed with glyme and these catalysts were used in conjunction with various salts such as zinc sulfate to increase catalyst reactivity. These catalysts have crystalline structures (See, Kuyper and Boxhom, Journal of Catalysis, 105, pp 163-174 (1987)).
U.S. Pat. No. 4,500,704 issued to Kruper, Jr., et al., teaches the use of DMC catalysts to produce polymers based on alkylene oxides and carbon dioxide as given in the table below. The amounts of cyclic, carbonates vary from 12% to 64% except for cis-cyclohexene oxide which forms no cyclic carbonate. The lack of formation of the cyclic carbonate from the reaction of cis-cyclohexene oxide and carbon dioxide may be related to steric factors in the formation of this bicyclic product and is not believed to be reflective of the products obtained with other alkylene oxides. The catalysts used in the '704 patent are glyme-zinc hexacyanocobaltate complexes which are known to those skilled in the art to have crystalline structures.
Data taken from U.S. Pat. No. 4,500,704 (See Table 1)Rxn.Rxn.CyclicTemp.TimeConversionCopolymercarbonatePolyetherExOxirane(° C.)(Hr)(%)(%)(%)(%)2propylene oxide354871761863ethylene oxide30843150401041-butylene oxide3548387117125propylene oxide258465851236propylene oxide404864662687propylene oxide802455064368cis-cyclohexene90243010000oxide
Hinz et al., in U.S. Pat. No. 6,762,278, teach the use of crystalline DMC catalysts having platelet-shaped structures which account for more than 30% of the particles. The improvement of Hinz et al. lies in the resulting polyether polycarbonates having narrower polydispersities than are obtained with other catalysts even where t-butyl alcohol (TBA) is used as a catalyst ligand. As can be appreciated by reference to the table below, the comparative catalysts in the '278 patent show polydispersities greater than 2.37. The polyether polycarbonates of the inventive examples of the Hinz et al. '278 patent have polydispersities less than 1.8. The formation of propylene carbonate is discussed in some of the examples of the '278 patent; however, the amounts are not given.
Data taken from U.S. Pat. No. 6,762,278*ComparativeInventiveexamplesPolydispersityexamplesPolydispersity12.9911.6322.5321.6933.8531.7342.3741.6253.3651.1665.5261.2671.3981.4691.58*Data from examples 1-9.
S. Chen et al. report the use of several DMC catalysts to prepare polyether polycarbonates, and they find cyclic carbonate contents ranging from about 13% to about 17% (See, Table 4 of S Chen et. al, J. Polymer, 45(19) 6519-6524, (2004)). The amounts of propylene carbonate agree with the ranges reported in U.S. Pat. Nos. 4,500,704 and 4,826,953. Although the authors do not report whether the catalysts that they used had crystalline or amorphous structures, the glyme-modified (1,2-dimethoxyethane) is generally accepted by those in the art to have a crystalline structure and all of the catalysts used in Chen's study gave cyclic carbonates in the same range.
Data taken from S. Chen et. al*CyclicComplexing agentcarbonate (%)1,2 Dimethoxyethane13.12-Methoxy Ethanol12.51-Methoxy-2-Propanol14.6THF16.2PPG 100015.6t-Butanol14.2None16.5*Data from Table 3 of S. Chen et al. See Table 1 of Chen et al. for additional data.
U.S. Pat. No. 6,713,599, issued to Hinz et al., teaches the addition of a sterically hindered chain transfer agent capable of protonating the polyol to reduce the amount of high molecular weight tail in a DMC catalyzed polyol production process. The invention of the Hinz et al. '599 patent also appears to improve the polydispersity.
Data taken from U.S. Pat No. 6,713,599ExamplePolydispersityAdditive11.31TBA21.36TBA31.412,4,6-tri-t-butyl phenol41.39phenol51.47catechol61.54di-t-butylbenzoic acidComparative 11.73noneComparative 21.99dipropylene glycolComparative 3*1.12difluorophenolComparative 4*1.11waterCompatative 52.12Low CatalystComparative 61.59Poor catalyst*Run Deactivated
However, a disadvantage of the Hinz et al. '599 patent lies in the necessity of adding a monofunctional initiator. As known to those skilled in the art, monofunctional materials cause deterioration in polymer properties when those materials are converted to polyurethanes.
Therefore, a need continues to exist in the art for polyether carbonate polyols containing a lower level of cyclic carbonate by-products than is achievable by the methods currently known in the art.